In an RFID communication processing device, a process in which a command is transmitted to a tag such that a carrier wave having a constant frequency is modulated while continuously transmitted and a process of transmitting an unmodulated carrier wave (called a Continuous Wave or CW for short) to receive a response from the tag are alternately performed. The tag that receives the command responds to the command by changing impedance of a circuit thereof. A signal (reflected wave) in which a reply signal with respect to the command is overlapped with the unmodulated carrier wave is sent back to the communication processing device by response operation of the tag, and the reply signal is decoded.
In RFID system installed in a production site or a physical distribution site, it is necessary to conduct communication with the tag within a previously-fixed communication range. However, depending on a wavelength of a radio wave used, sometimes the radio wave reaches a position beyond a necessary distance. Sometimes the radio wave is guided to a distant tag such that the radio wave is reflected by a floor or a wall of the site. As a result, the tag that exists in a position in which necessity of communication is not generated responds to the command, which results in a risk of generating a problem in information processing. FIG. 4 illustrates a specific example.
In the example of FIG. 4, antennas A1 and A2 of the communication processing device are disposed on sides of paths B1 and B2 parallel to each other, and information is read from a tag T attached to a vehicle C running on the paths B1 and B2 through the antennas A1 and A2. In the example of FIG. 4, it is necessary that only the antenna A1 corresponding to the path B1 on which the vehicle C is located can conduct communication with the tag T. However, the radio wave from the antenna A2 of the adjacent path B2 also reaches the tag T, and the tag T responds to the command from the antenna A2, which results in a risk of reading and writing false information.
In order to solve the above problem, there is a method for measuring a distance from the antenna to the tag to determine whether the measured distance is included in a communication range. A measuring method in which a phase shift of the reflected wave from the tag to the carrier wave transmitted from the antenna is utilized is also proposed as the method for measuring the distance from the antenna to the tag.
For example, in the description of Patent Document 1, there is described that the carrier waves of two ways having different frequencies are sequentially transmitted, an amount of change of the phase of the reflected wave from the tag is detected in each frequency, and the distance to the tag is fixed by a calculation in which a difference between the amounts of change is utilized.
In the description of Patent Document 2, there is described that communication is conducted with the tag attached to a moving object (train) that passes through a fixed path (railway track) using two antennas, a phase difference between reply signals that are received from the tag by the antennas is fixed, and a distance ΔL is fixed using the phase difference and a wavelength of the reply signal. Additionally, in the description of Patent Document 2, there is described that a hyperbolic curve, which is constructed by points at which a difference between the distances from the antennas becomes ΔL, is set by utilizing the distance ΔL that corresponds to a difference between a distance L1 from one of the antennas to the tag and a distance L2 from the other antenna to the tag, and the position of the tag is specified using the hyperbolic curve and a distance from a tag reader to the movement path of the tag (see paragraphs 0057 to 0061 and FIG. 4 of Patent Document 1).
Patent Documents
Patent Document 1: Japanese Patent No. 4265686
Patent Document 2: Japanese Unexamined Patent Publication No. 2011-37371
Problems Solved by the Disclosure
A method for detecting an amount of phase shift of the reflected wave from the tag to the carrier wave transmitted from the antenna and measuring the distance to the tag using the detected amount of phase shift and the wavelength of the carrier wave is also conceivable as a method that is simpler than the inventions described in Patent Documents 1 and 2. However, in the method, it is assumed that the radio wave transmitted from the antenna returns to the antenna through the path that becomes a length of one period or less there and back. That is, the distance up to a half period of the carrier wave can be measured by the method.
However, it is hardly considered that the condition is satisfied in the case that the carrier wave having the short wavelength is used, so that it is difficult to put such a method into practical use.
For example, in the case that the radio wave of 1 GHz in a UHF band is utilized, the carrier wave has the wavelength of about 30 cm, and this kind of radio wave has intensity to which the tag can react even if the radio wave reaches the position about 10 m away from the antenna. Generally, a communication distance is set in meters by utilizing the characteristic. The specifications cannot be applied to the distance measurement in which the phase difference between the signals having wavelengths of about 30 cm is used.
In view of the above problem, an object of the present disclosure is to implement measurement that can deal with the setting of the actual communication process such that the carrier wave with which an intermediate frequency is overlapped is transmitted during a cycle in which the reflected wave is received from the tag and such that the distance is measured by utilizing the phase shift of the intermediate frequency.